Paizo even wrote a FAQ for the lance:
Power Attack: If I am using a two-handed weapon with one hand (such as a lance while mounted), do still I get the +50% damage for using a two-handed weapon?
Yes.
Keep in mind the pathfinder rules are oddly specific at times. The rules state that while mounted, you can wield a lance with one hand.
If the rules would have said you'd treat it as a one-handed weapon, you'd use this FAQ instead:
Weapons, Two-Handed in One Hand: When a feat or other special ability says to treat a weapon that is normally wielded in two hands as a one handed weapon, does it get treated as one or two handed weapon for the purposes of how to apply the Strength modifier or the Power Attack feat?
If you're wielding it in one hand (even if it is normally a two-handed weapon), treat it as a one-handed weapon for the purpose of how much Strength to apply, the Power Attack damage bonus, and so on.
Covent wrote:Mounted combat FAQ is another bad knee jerk due to ragelancepounce
Nah, this was the FAQ to deal with ragelancepounce:
Lance: If I have the pounce ability and I charge with a lance, do my iterative lance attacks get the lance's extra damage multiplier from charging?
No, it doesn’t makes sense that those iterative attacks gain the damage bonus. To make that second attack, you have to pull the lance back and stab forward again, and that stab doesn’t have the benefit of the charge’s momentum. (The Core Rulebook doesn’t state that you only get the damage multiplier on the first attack with a lance because when the Core Rulebook was published, there was no way for a PC to charge and get multiple attacks with a weapon in the same round, so that combination didn’t need to be addressed.)